关于Basic程序解释器及编译原理的简单化(1)--词法分析和代数式求值

关于Basic程序解释器及编译原理的简单化(1)--词法分析和代数式求值(5)                 break;
            }
            for (t=*h-1;t>0;--t)  *r=(*r)*ex;
            break;
    }
}

/* reverse the sign */
void unary(char o,int *r)
{
    if (o=='-')  *r = -(*r);
}

/* find the value of a variable */
int find_var(char *s)
{
    if (!isalpha(*s))  {
        serror(4);  /* not a variable */
        return 0;
    }
    return variables[toupper(*token)-'A'];
}

/* display an error message */
void serror(int error)
{
    char *e[] = {
        "syntax error",
        "uNBAlanced parentheses",
        "no expression present",
        "equal sign expected",
        "not a variable",
        "label table full",
        "duplicate label",
        "undefined label",
        "THEN expected",
        "TO expected",
        "too many nested FOR loops",
        "NEXT without FOR",
        "too many nested GOSUB",
        "RETURN without GOSUB"
    };

    printf ("%s\n",e[error]);